Oil prices have risen, supported by expectations that sanctions on Iran would tighten global supplies, but futures posted a weekly drop as a slump in stock markets and concerns about trade wars clouded the fuel demand outlook.

Brent crude futures rose 73 US cents, or one per cent, to settle at $US77.62 a barrel.

The global benchmark marked a weekly loss of about 2.7 per cent and is down about $US10 in three weeks.

US West Texas Intermediate crude futures rose 26 cents, or 0.4 per cent, to end at $US67.59 a barrel.

It posted a weekly loss of about 2.3 per cent.

Prices got some support when two sources said on Friday Iraq will stop trucking crude oil from its northern Kirkuk oil field to Iran in November to comply with US sanctions.

Washington has said it wants to reduce Iranian oil sales to zero, although this looks unlikely.

Still, many buyers, including Iran's biggest customer, China, appear to be falling in line, forcing Tehran to store unsold oil on tankers.

"If you move forward and see people playing by the rules, which I don't believe ever really happens, you'll see supply come off and we could run into an issue later," said Michael McAllister, director of equity research at MUFG Securities.

A global collapse in equities has weighed on oil markets.

Financial markets have been roiled by the US-China trade war, a rout in emerging market currencies, rising interest rates and economic concerns in Italy.

There are also signs of a slowdown in global trade, with container and bulk freight rates dropping.

"If global demand contracts more than we think, that doesn't bode well for oil demand. GDP and oil demand are pretty well-correlated," said Stewart Glickman, an energy equity analyst at CFRA Research.

The recent market turmoil and forecasts for an increase in supply have caused investors to pull back on bets on higher oil prices.

Hedge funds cut their bullish bets on US crude to the most in more than a year, according to US Commodity Futures Trading Commission figures.

"The market in the fourth quarter could be shifting towards an oversupply situation as evidenced by rising inventories over the past few weeks," Adeeb Al-Aama told Reuters.

Saudi Energy Minister Khalid al-Falih said there could be a need for intervention to reduce oil stockpiles.

US crude production is soaring, boosted by technological advances. Output this year is forecast to break the annual record in 1970.

US energy firms added oil rigs for a third straight week, keeping the rig count at its highest in over three years, General Electric Co's Baker Hughes energy services firm said. Declining productivity in some shale fields has forced companies to drill more to keep output growing.